When you fulfill a commission that has not been matched you are directed to the matching history screen.
The name of the contributor is automatically put into the contributor field and if a planned for or published date is entered in the commission then that be inserted into the date field(s), otherwise RCS will input a date range of the last month.
Once you've found the relevant content to match to the commission click on Exit. Then when prompted, as shown below, click Yes to confirm the action. The commission is fulfilled and the content linked to it.